1. Field of the Invention
The present invention relates to an optical information record/reproducing apparatus for optical information recording and reproduction by optical beam irradiation, and more particularly to an optical information record/reproducing apparatus for effecting recording and reproduction in a parallel manner, utilizing plural light beams.
2. Related Background Art
In a conventional optical information record/reproducing apparatus for magnetic or magnetooptical information recording and reproduction, a verifying operation is conducted, after information recording, in order to confirm whether the information has been properly recorded. The verifying operation is usually conducted, after the information recording on an information track during a rotation of the information recording medium, by reproducing the thus recorded information during a next rotation of the recording medium. However, this method, requiring two steps for information recording, suffers from a slow recording speed, and an improvement in this respect has been desired.
Therefore, for reducing the time required for such verification and thus achieving high-speed recording, there has been proposed a direct verification method of effecting the verifying operation on real-time basis. Such direct verification is generally conducted by irradiating the information track of the information recording medium with two light spots positioned on the front and back thereof, recording the information with the preceding light spot and immediately reproducing the recorded information with the following light spot and comparing the reproduced information with the information to be recorded.
Another drawback associated with the conventional optical information record/reproducing apparatus lies in its slow data transfer rate. For resolving this drawback, there has been proposed a method of irradiating plural tracks of the recording medium simultaneously with plural light spots, thereby effecting information recording or reproduction in a parallel manner on plural tracks. Such optical information record/reproducing technology utilizing plural light spots has taken on great importance.
For forming plural light spots on the recording medium, there have been proposed various methods, including a method of dividing the light from a single light source into plural light beams by means of an optical element such as a diffraction grating, but, among such methods, considered particularly superior, is a method of utilizing plural light sources, in view of the advantage of independent intensity control of the light spots formed on the information recording medium.
In case of employing plural light sources, the optical paths are generally so designed that the plural light beams constitute a small angle therebetween and are focused, through a single objective lens, onto the information recording medium. Due to the image angles of the light beams at the entry into the objective lens, the focused points of the light beams on the recording medium are spatially separated, thereby forming plural light spots in a parallel manner. In the tracking control or focusing control, the plural light spots, being focused through a single objective lens, can be moved in the tracking or focusing direction while retaining a constant, mutual positional relationship by the movement of the objective lens in a corresponding direction. In such a configuration, the plural light spots irradiate a track or mutually separated tracks, thereby effecting information recording or reproduction in a simultaneous manner on the track or tracks. In such case the servo error signal for automatic focusing or tracking control can be obtained from the reflected light from any one of the light spots or from reflected lights from plural light spots.
Plural light sources can also be realized by a method of employing plural semiconductor laser diodes and combining light beams, for example, with half mirrors, or by employing a method of utilizing a semiconductor laser diode array.
On the other hand, in a reproducing optical system for reproducing the information by detecting the light reflected by the recording medium, the reflected light of the plural light spots have to be individually detected. In such detection, since the light beams respectively have small image angles, they have to be focused by condenser lenses for achieving spatial separation on the detector. Consequently, photodetectors are so positioned, in a divided manner, as to receive respective light beams in spatially separate focus positions, and the information is reproduced from the thus obtained detection signals.
The plural light sources may be realized, for example, as explained above, by employing plural semiconductor laser diodes and by combining the light beams thereof with suitable optical elements, such as half mirrors or dichroic mirrors. But, such a configuration is not preferable as it is significantly inferior in productivity to the optical head using a single light spot, due to the increased number of components, the increased number of positions requiring optical adjustments, and the difficulty of making optical adjustments.
Also, such a configuration is extremely susceptible to the positional aberrations, even by very small amounts, of the optical components from their adjusted states, because the positional precision between the photodetector surface and the light spot to be detected thereon has to be within a range of several microns. The use of plural semiconductor laser diodes is disadvantageous, because the number of associated components (for example collimating lens) increases with the number of such laser diodes, so that the number of independently factors leading to such positional aberrations also increases.
On the other hand, the detection optical system for detecting the servo error signal generally detects the displacement or deformation of the light spot for the focusing control with a divided photodetector as a variation in intensity, and the variation in the intensity distribution of the light spot for tracking control with a divided photodetector. In either case, however, the light spot has to be enlarged to a certain size, because the width of the dividing lines on the light-receiving face of the photodetector has a certain lower limit the (light spot diameter has to be somewhat larger than the width of the dividing lines), and in order to facilitate the positional adjustment of the photodetector. For this purpose there is, for example, employed a method of extending the focal length of the detected light condensing lens and positioning the photodetector out of the focal position of the condensing lens. However, as the plural light beams are only separated by angles as small as several minutes to several tens of seconds, the spatial separation on the photodetector becomes difficult because of interference with the adjacent light spots, if the detection is conducted at a position where the light beam diameter is relatively large.
For avoiding this difficulty it is conceivable to expand the angles of the plural light beams, but such expansion is difficult because of (1) an increased separation of the light spots on the recording medium, (2) increased aberrations due to an increased image angle at the entry into the objective lens, and (3) significantly different incident angles, among the light beams, into the dielectric film with a polarizing property.
If the recording medium is a rotating disk, the curvature of the information track is different between an outer part and an inner part of the disk. Thus, direct verifying operation is performed by irradiating the same information track with plural light spots and conforming the information, recorded by a preceding light spot, by a succeeding light spot, if the distance of the light spots is expanded as in the foregoing case (1), even when one of the light spots is properly positioned on the information track, the other tends to be displaced from the center of the information track. For this reason, it is difficult, particularly on an optical disk, to expand the distance of the light spots thereon.
In general, it is necessary to find a compromise between the above-mentioned requirements, and there is selected a configuration with a reasonably sized photodetector not requiring difficult positional adjustment and not showing significant loss in the non-sensitive portions thereof. For this reason, the light spots on such a photodetector are considerably close and are spatially separated only to a marginally acceptable level. In such a situation, the spatial separation of the light spots is realized only when the focusing servo loop is closed. But, if the recording medium is somewhat significantly defocused with respect to the objective lens, the light spot on the photodetector will also be enlarged by such defocusing, and may overlap with the adjacent light spots on the photodetector. In such a case, each divided portion of the photodetector will receive a leaking light of the adjacent light spots, whereby the proper light amount balance on the divided portions of the photodetector is destroyed and a proper focus error signal can be no longer obtained. Also the S-shaped curve of the auto focusing control is deformed, thereby leading to inconveniences such as a narrower linear range and a sudden change of S-shaped sensitivity, eventually giving rise to the following drawbacks.
At first, the pull-in of the focusing servo operation becomes difficult or impossible, depending on the magnitude of the vibration of the recording medium. Also when an external perturbation, such as a shock or a vibration induces defocusing, the S-shaped curve will be deformed due to the interference of the adjacent light spots on the photodetector, thus easily inducing, for example, an oscillation of the focusing actuator and eventually hindering the focusing servo operation, even in a situation where such hindrance is not likely to occur. Such mutual interference of the light spots can be prevented by an expansion of the distances of the light spots, but such expansion is limited by another condition as explained before. For these reasons, there have remained certain problems to be solved, relating to the focusing control of the light spots.